The present invention relates to a fuel cell, and more precisely a fuel cell using a polymeric membrane as the electrolyte.
Fuel cells are chemical generators of electric energy in the form of direct current; in other words, they convert the free energy of reaction of a fuel (for example a gaseous mixture containing hydrogen, or a light alcohol such as methanol or ethanol) with an oxidant (for example air or oxygen) without its complete degradation to thermal energy, and therefore without being submitted to the limitation of the Carnot cycle. To achieve the desired conversion of chemical energy to electrical energy, the fuel is oxidised at the anode of the cell, with the concurrent release of electrons and H+ ions, while the oxidant is reduced at the cathode, where H+ ions are consumed; the two poles of the generator must be separated by a suitable electrolyte, allowing a continuous flow of H+ ions from the anode to the cathode, at the same time hindering the transfer of electrons from one pole to the other, thereby maximising the electrical potential difference between the two electrodes. This potential difference represents in fact the driving force of the process itself. The fuel cells are considered as an excellent alternative to the traditional systems of electric generation; especially in view of their extremely favourable environmental impact (absence of polluting emissions and noise, formation of water as the only by-product), they are used both in the field of stationary power generation of various sizes (electrical power stations, back up power generators, etc.) as well as in the field of mobile applications (electric vehicle applications, generation of automotive energy or auxiliary energy for space, submarine and naval applications).
The polymeric membrane fuel cells offer, compared with other fuel cells, further advantages, due to their fast startup an quick achievement of the optimum operation conditions, the high power density, the intrinsic reliability connected both to the lack of moving parts and to the absence of corrosion phenomena and severe thermal cycles; in fact, among all the fuel cells of the prior art, the polymer electrolyte fuel cells exhibit the overall lowest operating temperature (usually, 70-100xc2x0 C.).
The polymeric electrolyte used for this purpose is an ion-exchange membrane, and more precisely a cation-exchange membrane, that is a chemically inert polymer, partially functionalised with groups capable of undergoing acid-base hydrolysis leading to a separation of electric charge; said hydrolysis consists more precisely in the release of positive ions (cations) and in the formation of fixed negative charges on the polymer constituting the membrane. Porous electrodes are applied on the surface of the membrane, which allow for the reactants to flow therethrough up to the membrane interface. A catalyst is applied on said interface on the electrode and/or on the membrane side, such as for example platinum black, which favours the corresponding half-reaction of fuel oxidation or oxidant reduction. This arrangement provides also for the continuous flow of cations when a potential gradient is established between the two faces of the membrane and the external electric circuit is concurrently closed; being the H+ ion the transported cation in this case, as previously mentioned, the potential difference generated upon feeding a species with a lower electrochemical potential at the anode, and a species with a higher electrochemical potential at the cathode, causes protonic conduction across electron flow (i.e. electric current) across the external circuit, as soon as the latter is dosed.
The protonic conduction is an essential condition for the operation of a fuel cell and is one of the decisive parameters to assess its efficiency. An insufficient protonic conduction causes a remarkable drop in the potential difference at the poles of the cell (cell voltage drop) once the electric circuit is closed on the external resistive load which exploits the produced electric output. This, in turn, causes an increased degradation of the energy of reaction to thermal energy and the consequent decrease of the fuel conversion efficiency.
Several cation-exchange membranes, offering optimum protonic conduction characteristics, are available on the market and are widely used in industrial fuel cells, such as for example those commercialised under the trademark Nafion(copyright) by Dupont de Nemours, U.S.A., Gore Select(copyright) by Gore. U.S.A., Aciplex(copyright) by Asahi Chemicals, Japan. All these membranes are negatively affected by an intrinsic process limitation associated with their operation mechanism: being the separation of electric charge which enables the protonic conduction set by a hydrolysis mechanism, such membranes develop their conductivity only in the presence of liquid water. Although the formation of water is an intrinsic consequence of the operation of a fuel cell, its extent results almost always insufficient to maintain the correct hydration state of the membrane, especially when operating at a sufficiently high current density.
Operation at high current density in fact involves a decrease in the investment costs for a given power output, but also a decrease in the energy efficiency and the generation of a higher amount of heat. The large amount of heat generated in a fuel cell operating at a current density of practical use (for example between 150 and 1500 mA/cm2) must be efficiently removed to permit the thermal regulation of the system, not only in view of the limited thermal stability of the ion-exchange membrane, usually unfit for operation above 100xc2x0 C., but also to reduce as much as possible the evaporation of the produced water and its consequent removal by the flow of inerts and unconverted reactants from the cell. Furthermore, as the voltage at the poles of a single fuel cell is too small to allow a practical exploitation, said cells are usually connected in electrical series by means of bipolar junctions and assembled in a filter-press arrangement feeding the reactants in parallel, as illustrated in U.S. Pat. No. 3,012,086. In such a fuel cell battery arrangement, usually called a xe2x80x9cstackxe2x80x9d, the problem of heat removal is amplified with respect to the case of a single cell, wherein it is possible to take advantage of the thermal convection through the external walls. For this reason, all the designs of prior art fuel cells provide suitable hydraulic circuits for the removal of heat by thermal exchange with a circulating fluid; such fluid may be fed inside serpentines formed in the bipolar plates or in appropriate sections intercalated between single cells in electrical connection therewith; both solutions further complicate the construction of the stacks, increasing weights and volumes, thereby reducing the power density, a parameter whose maximisation is highly desired especially in the case of mobile applications.
A less burdensome solution under this aspect is described in the PCT patent application no. WO 98/28809, wherein the cooling fluid is circulated in a peripheral section of the bipolar plate adjacent to the active surface of the cell; however, in this way a transversal temperature profile is obtained with the central area of the membrane operating at a temperature higher than that of the peripheral area, thereby establishing a thermal gradient which is potentially very dangerous for the integrity of the membrane itself.
Finally, even if the extent of the heat removal needed to set the system temperature below 100xc2x0 C. appears to be achievable although quite demanding, the concurrent water drain from fuel cell stacks remains too high for the produced water to maintain a sufficient hydration level of the membranes alone; the stack designs of the prior art have therefore introduced a second auxiliary system, in addition to the cooling system, which provides for injecting the required additional amount of water into the generator. This circuit generally provides for pre-humidifying the reactants at the inlet of the anode and cathode compartments of the fuel cells, for example by bubbling in liquid water or by diffusion of water vapour though suitable membranes in auxiliary cells. Also this second circuit involves an evident increase in weight, volumes and investment costs; moreover, the quantity of water to be fed to the system must be strictly controlled as an excess of liquid in the cell compartments would lead to the dramatic consequence of blocking the access of the gaseous reactants to the surface of the electrodes. The only possibility to achieve a calibration, albeit indirect, of the water supplied by the above system is acting on the temperature of the water itself and thus on its vapour pressure. This in turn brings to the need of thermostating the humidification system of the fuel cell stacks, further complicating the construction design.
A more advantageous solution to ensure a suitable water supply to the reactant flow is disclosed in the European Patent Publication No. 316 626 where it is described the humidification of said flow through injection of atomised water thereto, for example by means of a ultrasonic aerosol generator. This solution partially mitigates the need of cooling the stack by a burdensome auxiliary heat exchange circuit, as part of the water fed thereto is vaporised inside the cell, thereby removing a substantial amount of heat. The system however is negatively affected by a basic drawback represented by the construction complexity associated with the aerosol generator which, besides being expensive, consumes a certain portion of the electric output generated by the fuel cells.
In addition, the time of permanence of water in the cell is too short to ensure at the same time the humidification of the membrane and the cooling of the stack without recurring to auxiliary circuits, especially at a high current density and with stacks comprising a high number of cells.
Furthermore, the humidification of the reactants or the addition of atomised water prior to sending said reactants to the inlet manifold may cause some water condensation or droplet formation therein, having the consequence of feeding an excess amount of water to some cells of the stack (typically tho closer to the reactants inlet) and an insufficient amount to some other cells (typically those farther from the reactants inlet).
The present invention consisting a fuel cell stack comprising a reticulated electrically and thermally conductive material interposed between the bipolar plate and the electrodic surface as described for example in U.S. Pat. No. 5,482,792, wherein humidification of the reactants and thermal control are obtained by a single-circuit direct injection of a suitable flow of water which partially evaporates inside the reticulated material exploiting its high surface and its thermal conductivity which allows an efficient extraction of heat from the electrodes.
In one embodiment of the invention, the injection point of the water in the gaseous flow is positioned downstream the reactant inlet manifold.
In another embodiment, said injection point is positioned in correspondence of the periphery of the reticulated material, in areas physically separated from the ones where the reactants are fed.
In another embodiment, water is injected in correspondence of depressions formed inside the reticulated material.
In another embodiment, water is injected in correspondence of serpentine-shaped depressions provided inside the reticulated material and running along the whole surface of the same.
In another embodiment, water is injected in correspondence of offset double comb-shaped depressions provided inside the reticulated material.